CROSS REFERENCE TO RELATED APPLICATION
Attention is directed to U.S. application Ser. No. 614,201 entitled "Removable Process Cartridge for Electrostatographic Reproducing Apparatus" filed on May 25, 1984 in the name of Hoppner et al and commonly assigned to the assignee of the present invention.
The present invention relates to a electrostatographic reproducing apparatus and more particularly to a mechanical development control system for use in such apparatus.
In the electrostatographic reproducing apparatus commonly in use today, a photoconductive insulating member is typically charged to uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the usual document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas in the photoconductive insulating area to form a powder image on the photoconductive area. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Alternatively, development may be carried out with a single component developer comprising toner particles which are made from a ferromagnetic material in a suitable resin binder.
One of the most common development techniques for use with both two component (carriers+toner) and single component developer (toner) involves use of a magnetic brush development device. In such a device the developer material is presented to the imaging member by means of a rotatably mounted cylindrical sleeve which has a elongated magnet stationarily positioned with the sleeve to form a magnetic brush of the developer particles. Typically, the rotatable magnetic brush forms a development nip with the image bearing member. During development the copy quality obtained in the developed image depends on the amount of toner material on the developer roll. If insufficient toner material is on the developer roll no development may take place or development of very low density may be perceived. As the amount of toner material present on the roll is increased, it reaches a point where the developed darkness obtained dramatically increases even in fact, provides an overdevelopment of the electrostatic latent image. By overdevelopment we mean that fuzzy heavy lines are produced since the magnetic brush, while it has sufficient time to develop the electrostatic latent image, has insufficient time or developer contained in a development zone to clean up or scavange the background portions of the image. With a slight further increase in the amount of toner present on the development roll the optimum level of copy quality can be obtained. This is a relatively broad range of toner amount that may be present on the roll that provides both adequate development by the toner as well as the scavanging of toner particles initially placed in the background area on the imaging member. Further increases in the amount of toner present on the developer roller beyond this optimum range may lead to spillage and contamination of the machine throughout.
Accordingly, copy quality of the final output copy in such automatic reproducing apparatus depends on having a constant amount of toner on the developer roll. If the amount of toner is below this constant optimum amount the development zone formed within the nip between the developer roll and the photoreceptor may be too small or the amount of toner present there may be insufficient to perform both the development function and the scavanging function. Thus, if the amount of toner runs low poor or thin copy quality is perceived. poor develpment and low scavanging results in fuzzy images and background deposits on the final copy. On the other hand if too much toner is present the possibility of it leaking out of the development system and contaminating the machine is very high. Accordingly there is a desire to maintain the amount of toner present in the development nip within a given quantity range at all times. Thus when toner is removed during the development process additional toner needs to be supplied to the development zone to insure adequate development.
Furthermore to insure proper development and cleaning in the development zone it has been found desirable to extend the development zone from the nip in order to provide sufficient toner as well as development time in which to both develop the image and scavange the background portions of the image. Accordingly toner is pumped into the development zone so as to form a wedge shaped mass of toner above the small nip between the developer roll and the image bearing member. This provides the additional advantage in those electrostatographic reproducing machines which employ a two-cycle process wherein one or more units of the machine perform more than one function, for example, as in the present invention the developing unit may also provide the function for cleaning. This is achieved by forming and developing the electrostatic latent image during the first pass, transferring the image to a copy sheet and cleaning any residual image in the second pass. With the extended development zone during the cleaning cycle when there is no potential applied to the photoconductor a more satisfactory scavanging of the photoreceptor and thereby cleaning is achieved.